The Bayer process is the most widely used method for the recovery of alumina from aluminiferous ores, such as bauxite. The Bayer process involves digestion of bauxite in a caustic solution, generally sodium hydroxide, usually at elevated temperatures and pressures. Digestion of the ore results in a slurry consisting of a sodium aluminate solution and a caustic-insoluble residue, commonly referred to as "spent bauxite" or "red mud". After digestion is completed, the slurry is cooled and the sodium aluminate solution, referred to as "pregnant liquor" is separated from the spent bauxite. The pregnant liquor is then seeded and/or agitated, and product alumina hydrate is recovered from the liquor by precipitation. The product is separated from the liquor by settling and/or filtration, washed to remove entrained liquor and then dried or calcined. The liquor from which a portion of the dissolved alumina has been removed by precipitation is referred to as "spent liquor" and is recycled to the digestion step after adjustment of its caustic content by evaporation and/or addition of replacement or make-up caustic. The red mud or spent bauxite is washed to recover most of its entrained liquor content and is then removed from the Bayer process.
In the Bayer process practically all process streams are contaminated with impurities, such as sodium chloride, sulfur containing compounds and sodium carbonate. Some of these impurities originate from the bauxite feed material, others are introduced with the caustic solution, settling and filter aids and other additives employed for the control of metallic impurities in the product alumina hydrate.
Since for economical reasons most of the Bayer process streams are recycled the impurities in these streams can steadily increase and rapidly reach levels at which the yield of alumina will be significantly affected. Additionally, increased impurity levels in the streams can cause other problems as well in the processing of bauxite, for example excessive foaming, density and viscosity increase and pronounced scaling on the walls of vessels and pipes used for treating and transporting these streams.
Several methods are in use for the control of impurity levels in Bayer process liquors or streams. One method utilizes, for example, high purity, sodium chloride-free caustic for digestion of the bauxite; evaporative desalting of Bayer streams is also employed, this results in the precipitation of Na.sub.2 CO.sub.3. If the process stream also contains Na.sub.2 SO.sub.4, evaporative desalting produces burkeite, which is a Na.sub.2 CO.sub.3 --Na.sub.2 SO.sub.4 double salt. Purification of the Bayer process streams by ion exchange removal of certain impurities has also been suggested. An alternative purification method involves blow-down or bleeding of a side stream which provides a certain degree of impurity control, however disposal of this side stream may present difficulties and results in the loss of valuable alumina and caustic.
The problem of maintaining the impurity level of Bayer streams at, or below a certain predetermined value is further compounded by strict environmental regulations which impose restrictions on the type and nature of Bayer process effluents which can be discharged into natural receiving bodies.
For example disposal of the spent bauxite slurry byproduct of the digestion generates a serious problem both from an environmental and from an economical point of view. In order to avoid contamination of natural bodies of water, the spent bauxite slurry is impounded, usually in large man-made ponds. This solution to the disposal problem, while it satisfies environmental concern, creates severe economic strains on alumina producers since the spent bauxite slurry contains large volumes of aqueous liquor which occupies a large impoundment area. To reduce the area for impoundment it is desirable to remove at least a portion of the aqueous liquor or effluent. This effluent, however, contains impurities, including dissolved aluminum and caustic values, the latter imparting undesirably high pH to the liquor. Also, there may be suspended solids in the effluent and consequently it does not lend itself to direct disposal into natural receiving bodies. Removal of the suspended solids and neutralization of the highly caustic effluent is economically prohibitive due to the large volumes to be treated. An economically feasible solution to this problem is the recycling of this effluent to the Bayer process. However, recycling of the effluent to the Bayer process returns impurities to the process streams, thus compounding the already existing purity control problems.
A process has now been discovered which simultaneously provides: (1) control of the impurity level of Bayer process streams, (2) allows, if desired, recycle of the spent bauxite disposal system effluent to the Bayer process without neutralization, (3) recovers valuable aluminum and sodium values and last, but not least, (4) generates an effluent containing only environmentally acceptable salts, such as sodium chloride and bicarbonate, thus allowing its direct discharge in natural receiving bodies. These results can be obtained by selecting a suitable blow-down stream from the Bayer process, carbonating this stream which has a pH in excess of 12 with carbon dioxide in a batch, continuous or semicontinuous manner at 40.degree.-100.degree. C until essentially all of its dissolved alumnia content is precipitated in the form of sodium dawsonite [NaAl(OH).sub.2 CO.sub.2 ] which also results in the recovery of 1 mole of Na/mole of Al and separating the dawsontie from the residual liquor having a pH of about 8.6-9. the dawsonite-free effluent containing only environmentally acceptable dissolved salts can be readily disposed of, for example, into natural receiving bodies, while the sodium dawsonite can be utilized in as-is condition or recycled to the Bayer process after thermal decomposition as sodium alumina.